Keratoconus is a disorder that causes progressive thinning and distortion of the cornea - the clear dome at the front of the eye. The cornea normally has a rounded shape, but in keratoconus the thinned area bulges forward to produce a cone like protrusion. This results in progressive distortion and reduced vision. Many keratoconus patients experience blurred distance vision, glare, light sensitivity, disturbed night vision, headaches and reading problems, Keratoconus may affect only one eye but more commonly occurs in both eyes. It affects people of all races and both sexes. Most patients develop keratoconus in their late teens to early twenties although it can begin at any age. Pellucid marginal degeneration (PMD) is a corneal thinning disorder related to keratoconus that primarily affects the inferior cornea.